The present invention relates to a system for controlling the shifts of an automatic transmission for vehicles and a process for executing the shifts. More particularly, the present invention relates to a shift control system for an automatic transmission, which is equipped with first and second transmission assemblies connected in series for executing a downshift, for example, in its entirety by shifting down the first transmission assembly to increase the gear ratio thereof and simultaneously shifting up the second transmission assembly to decrease the gear ratio thereof, and a process for executing such shifts.
There is heretofore known an automatic transmission having first and second transmission assemblies connected in series, the former of which is capable of effecting three-stage shifts of first to third speeds and the latter of which is capable of effecting two higher and lower shift stages. The automatic transmission of this kind can be set to totally six gear stages by switching the second transmission assembly to the higher and lower stages for each gear stage of the first transmission assembly.
In the shifts between those six gear stages or the shifts between the predetermined gear stages selected from the six stages, some shift is achieved by shifting up one of the first and second transmission assemblies and shifting down the other transmission assembly. In other words, the first and second transmission assemblies may be shifted in opposite directions to each other to execute the shift. In this case, the individual shifts are not timed if the transmission assemblies are individually controlled. Then, the driving feel is deteriorated by an increase in the shifting shocks, by an upshift at the second transmission assembly after a downshift at the first transmission assembly, or by a downshift at the first transmission assembly after an upshift at the second transmission assembly.
In order to eliminate such disadvantages, there has been heretofore proposed the following sequential control in case the automatic transmission is to be shifted up in its entirety by shifting the first and second transmission assemblies in opposite directions to each other. In the sequential control, a shift at the transmission assembly for shifts having a small change in the gear ratios is ended while a shift at the transmission assembly for shifts having a high change in the gear ratios. In case the first and second transmission assemblies are to be shifted in the opposite directions, there has also been proposed a shift control which has its shift ending points set coincident at each transmission assembly, as exemplified by Japanese Patent Laid-Open No. 31754/1987. According to this disclosure, after a command signal for an upshift at one transmission assembly has been outputted, a command signal for a downshift at the other transmission assembly is outputted simultaneously as the start of the actual upshift is detected. Moreover, the shifts at the individual transmission assemblies are simultaneously ended by controlling the application pressures of the frictional engagement elements of the transmission assembly to be shifted down, that is, by controlling the drains of the oil pressures from the frictional engagement elements.
In order to execute a high-grade sequential control for synchronizing the ends of the shifts at the individual transmission assemblies, however, the fluctuations of the engine torque or the various dispersions in the automatic transmission cannot be coped with merely by controlling the application pressures of the frictional engagement elements in one transmission assembly. Thus, there arises a problem that a discrepancy is inevitably caused at the end of the shifts of the individual transmission assemblies.
Here, the application pressures of the frictional engagement elements of each transmission assembly could be individually controlled by providing the transmission assembly with a linear solenoid valve. Then, there arises another problem that the weight and production cost are invited by the increased number of control devices.